AQA A-Level Biology: Exchange, Transport and Energy Transfers Revision Guide

Topics 3, 4 and 5 of AQA A-Level Biology (7402) contain some of the most content-heavy material on the specification. Topic 3 (Exchange) and Topic 4 (Genetic Information, Variation and Relationships) are examined on Paper 1, while Topic 5 (Energy Transfers) appears on Paper 2. All three can also appear on the synoptic Paper 3.

This guide works through the key content of each topic and highlights what examiners are looking for in your answers.

How These Topics Fit Into the Exam

• Paper 1 covers Topics 1--4. It is 2 hours long, worth 91 marks and accounts for 35% of the A-Level.
• Paper 2 covers Topics 5--8. It is also 2 hours, worth 91 marks and 35% of the A-Level.
• Paper 3 can draw on any content from Topics 1--8. It is 2 hours, worth 78 marks and 30% of the A-Level. This paper includes synoptic questions and the 25-mark essay.

Because these three topics span two papers, steady and cumulative revision is essential.

Topic 3: Exchange (Paper 1)

The underlying principle running through this entire topic is the relationship between surface area and volume.

Surface Area to Volume Ratio

As organisms increase in size, their volume grows more rapidly than their surface area. Simple diffusion across the body surface becomes insufficient, so large organisms require specialised exchange surfaces. These surfaces share common features: a large surface area, thin walls to minimise diffusion distance, and mechanisms to maintain steep concentration gradients (such as a good blood supply or ventilation).

Gas Exchange in Humans

The human gas exchange system centres on the lungs. Air enters through the trachea, which branches into bronchi and then progressively smaller bronchioles, terminating in clusters of alveoli. The lungs contain approximately 300 million alveoli, giving a combined surface area of around 70 square metres.

Alveolar adaptations are a classic exam question:

• Large total surface area -- maximises the rate of diffusion.
• Thin walls -- each alveolus and its adjacent capillary wall are one cell thick, giving a very short diffusion pathway.
• Good blood supply -- a dense capillary network maintains a steep concentration gradient.
• Ventilation -- breathing continuously replaces air in the alveoli, sustaining the gradient on the air side.

Gas Exchange in Fish

Fish use gills with gill filaments subdivided into lamellae. The key adaptation is countercurrent flow -- blood flows through the lamellae in the opposite direction to water. This maintains a concentration gradient along the entire length of the lamella, making oxygen uptake far more efficient than parallel flow would allow.

Gas Exchange in Insects

Insects use a tracheal system. Air enters through spiracles and passes into tracheae, which branch into tracheoles that penetrate directly to the tissues. During exercise, fluid is withdrawn from the tracheole ends to increase the gas exchange surface area.

Gas Exchange in Plants

Plants exchange gases through stomata, controlled by guard cells. When guard cells are turgid the stoma opens; when flaccid it closes. Xerophytic plants reduce water loss through adaptations such as sunken stomata, thick waxy cuticles, rolled leaves and reduced stomatal density.

Digestion and Absorption

The ileum (small intestine) is the principal site of absorption. Its inner surface is folded into villi, and the epithelial cells lining each villus have microvilli on their apical surfaces. This arrangement enormously increases the surface area available for absorption. The epithelium is just one cell thick, minimising the diffusion distance, and each villus contains a rich capillary network and a lacteal for the absorption of fats.

Glucose and amino acids are absorbed by co-transport with sodium ions. The sodium-potassium pump on the basal membrane actively removes sodium from the epithelial cell, maintaining a low intracellular sodium concentration. Sodium then enters the cell from the lumen via co-transporter proteins, carrying glucose or amino acids with it.

Mass Transport in Animals

Haemoglobin is a quaternary protein with four polypeptide subunits, each carrying a haem group that binds one oxygen molecule. The oxygen dissociation curve is sigmoid due to cooperative binding. The Bohr effect shifts the curve to the right when carbon dioxide concentration rises, so haemoglobin releases oxygen more readily in respiring tissues. Fetal haemoglobin has a higher oxygen affinity than adult haemoglobin -- its curve is shifted left, enabling it to pick up oxygen across the placenta.

The cardiac cycle consists of atrial systole, ventricular systole and diastole. Cardiac output equals heart rate multiplied by stroke volume. You should be able to interpret pressure-volume graphs and explain valve opening and closing.

Blood vessel structure relates to function: arteries have thick elastic walls for high pressure; capillaries are one cell thick for efficient exchange; veins have valves to prevent backflow.

Mass Transport in Plants

Xylem carries water and minerals upward, driven by transpiration. The cohesion-tension theory explains how hydrogen bonds between water molecules maintain a continuous column pulled upward as water evaporates from the leaves. Root pressure also contributes.

Phloem transports sucrose from sources to sinks by translocation. The mass flow hypothesis explains this: sucrose is actively loaded into sieve tubes, lowering water potential and drawing in water by osmosis. The resulting hydrostatic pressure drives flow towards the sink.

Required Practical: Dissection

You should be able to dissect a heart or lungs, identify key structures (ventricles, atria, valves, aorta, pulmonary artery, coronary arteries) and explain how structure relates to function.

Topic 4: Genetic Information, Variation and Relationships (Paper 1)

DNA, Genes and Chromosomes

A gene is a base sequence of DNA that codes for a polypeptide. Exons are coding sequences; introns are non-coding and removed during mRNA processing. The genome is the entire DNA of an organism. Different versions of a gene are alleles.

DNA Replication

DNA replication is semi-conservative. DNA helicase separates the strands by breaking hydrogen bonds. DNA polymerase adds free nucleotides to each template strand following base-pairing rules. The Meselson-Stahl experiment provided evidence for this model.

Protein Synthesis

Protein synthesis occurs in two stages.

Transcription takes place in the nucleus. RNA polymerase binds to the template strand of the gene and assembles a complementary mRNA molecule. In eukaryotes, the initial transcript (pre-mRNA) contains introns, which are spliced out to produce mature mRNA that leaves the nucleus.

Translation takes place at the ribosomes in the cytoplasm. The mRNA sequence is read in triplets called codons. Transfer RNA (tRNA) molecules, each carrying a specific amino acid, have anticodons that are complementary to the mRNA codons. As each tRNA binds at the ribosome, its amino acid is added to the growing polypeptide chain by peptide bond formation.

Genetic Diversity

Genetic diversity arises through:

• Mutations -- changes to the DNA base sequence that can create new alleles.
• Meiosis -- independent assortment and crossing over during prophase I produce genetically unique gametes.
• Random fertilisation -- the combination of any sperm with any egg generates enormous variation.

These mechanisms provide the raw material for natural selection and adaptation.

Species, Taxonomy and Biodiversity

A species is a group of organisms that can interbreed to produce fertile offspring. The three-domain system (Bacteria, Archaea, Eukarya) is based on molecular phylogenetics, particularly ribosomal RNA comparisons.

Species richness counts the number of species in a community. The index of diversity also accounts for relative abundance, giving a more meaningful biodiversity measure.

Required Practical: Investigating Diversity

Use random sampling with quadrats to investigate organism distribution and abundance. You should be able to calculate species frequency, percentage cover and an index of diversity. Understand when systematic sampling (such as a belt transect) is more appropriate.

Topic 5: Energy Transfers (Paper 2)

This is the most biochemically demanding part of the specification, covering the metabolic pathways of photosynthesis and respiration alongside energy flow through ecosystems.

Photosynthesis

Light-Dependent Reactions

These occur on the thylakoid membranes. Light energy absorbed by chlorophyll is used to:

1. Photolyse water -- splitting water into protons, electrons and oxygen.
2. Excite electrons along an electron transport chain, pumping protons into the thylakoid space.
3. Drive chemiosmosis -- protons flow through ATP synthase, producing ATP by photophosphorylation.
4. Reduce NADP -- electrons and protons combine with NADP at the end of the chain.

Light-Independent Reactions (The Calvin Cycle)

These occur in the stroma and have three stages:

1. Carbon fixation -- CO2 combines with ribulose bisphosphate (RuBP), catalysed by RuBisCO, forming glycerate-3-phosphate (GP).
2. Reduction -- GP is reduced to triose phosphate (TP) using ATP and reduced NADP.
3. Regeneration of RuBP -- most TP regenerates RuBP using ATP; a small proportion is used to synthesise glucose.

Limiting Factors

Light intensity, CO2 concentration and temperature can each limit the rate of photosynthesis. You should interpret graphs showing how the rate plateaus when another factor becomes limiting.

Required Practical: Chromatography

Use chromatography to separate photosynthetic pigments. Pigments travel different distances depending on their solubility in the solvent. Calculate Rf values and identify chlorophyll a, chlorophyll b, carotene and xanthophyll.

Respiration

Glycolysis

Occurs in the cytoplasm without oxygen. Glucose is phosphorylated using 2 ATP, split into two triose phosphates, and oxidised to pyruvate -- yielding a net gain of 2 ATP and 2 reduced NAD.

The Link Reaction

Pyruvate enters the mitochondrial matrix and is decarboxylated and dehydrogenated to form acetyl CoA, producing CO2 and reduced NAD.

The Krebs Cycle

Acetyl CoA combines with oxaloacetate to form citrate. Through decarboxylation and dehydrogenation reactions, citrate is converted back to oxaloacetate. Each turn produces 1 ATP, 3 reduced NAD, 1 reduced FAD and 2 CO2. The cycle turns twice per glucose molecule.

Oxidative Phosphorylation

This is the final stage and takes place on the inner mitochondrial membrane (the cristae). Reduced NAD and reduced FAD donate their electrons to the electron transport chain -- a series of carrier proteins embedded in the membrane. As electrons pass along the chain, energy is released and used to pump protons from the matrix into the intermembrane space, creating a proton gradient. Protons then flow back into the matrix through ATP synthase by chemiosmosis, driving the synthesis of large quantities of ATP. Oxygen acts as the final electron acceptor, combining with protons and electrons to form water. Without oxygen, the electron transport chain stops entirely.

Anaerobic Respiration

Without oxygen, reduced NAD accumulates. To regenerate NAD, pyruvate is converted to ethanol and CO2 (in yeast and plants) or lactate (in animals). This yields far less ATP than aerobic respiration.

ATP as the Universal Energy Currency

ATP releases a small, manageable amount of energy per hydrolysis -- enough to drive individual reactions efficiently. It is rapidly regenerated from ADP and inorganic phosphate. You should be able to explain why ATP is more suitable than glucose as an immediate energy source.

Energy Transfer Through Ecosystems

Energy enters most ecosystems as light and is fixed by producers during photosynthesis. The total energy fixed is gross primary productivity (GPP). Some of this is used by the producers themselves in respiration, and the remainder is net primary productivity (NPP): NPP = GPP - R.

Energy is transferred between trophic levels when organisms feed on one another. At each transfer, a significant proportion of energy is lost -- mainly through respiration, but also in waste products and parts of organisms that are not consumed. The efficiency of energy transfer between trophic levels is typically only 10--20%, which is why food chains rarely have more than four or five levels.

Farming practices can increase the efficiency of energy transfer to humans. Restricting the movement of livestock reduces energy lost through respiration. Controlling environmental temperature reduces energy spent on thermoregulation. Selective breeding and high-energy feeds also raise productivity. You should be prepared to evaluate the ethical and environmental implications of intensive farming methods.

Exam Tips for These Topics

• Learn metabolic pathways in sequence. Name specific molecules at each stage -- vague references to "energy being released" will not earn marks.
• Use precise terminology. Say "decarboxylation" not "carbon is removed." Say "reduced NAD" not "hydrogen carrier."
• Draw diagrams where appropriate. A labelled oxygen dissociation curve or Calvin cycle diagram demonstrates understanding clearly.
• Practise data interpretation. Questions frequently include graphs on dissociation curves, photosynthesis rates and transpiration rates.
• Link structure to function. Whether describing an alveolus, a villus or a mitochondrion, always explain how the structure is adapted for its role.

Prepare with LearningBro

Targeted practice on these topics will build your confidence and sharpen your exam technique.

• AQA A-Level Biology: Exchange and Genetics -- focused practice on Topics 3 and 4, covering exchange surfaces, mass transport, DNA replication and protein synthesis.
• AQA A-Level Biology: Energy and Ecosystems -- questions on photosynthesis, respiration and energy flow through ecosystems.
• AQA A-Level Biology: Energy Transfers in Depth -- a deep dive into the biochemistry of photosynthesis and respiration, including metabolic pathways and required practicals.
• AQA A-Level Biology -- comprehensive coverage of the entire AQA specification across all eight topics.

Start working through exam-style questions on these topics now, and review your answers against the mark scheme to identify gaps in your knowledge.